Just a short and silly note to point out that Canada no longer has piggy-backed sales taxes. PEI and Québec used to calculate provincial sales taxes on the sub-total of sale + the GST (hence the term piggy-back). This led to annoying special code. Well, annoying to programmers at least.
But Québec moved to PST+GST on the January 1st 2013, and PEI moved to HST on April 1st, 2013.